Forget Jenny Craig. Hit the Drive-Thru.

Christine Dougherty, a 27-year-old business consultant in Pensacola, Fla., thinks so. “I don’t like to cook, and I wanted to be realistic without changing my lifestyle too much,” Ms. Dougherty said. She began replacing her usual fast-food lunch or dinner with meals from the Fresco menu at Taco Bell, which consists of seven items — including burritos and tacos — each with less than 9 grams of fat, compared with, say, 30 grams of fat in the Stuft beef burrito on the regular menu.

Ms. Dougherty said that she ate there five to eight times a week, exercised more and — over two years — lost 54 pounds. By December 2009, she was the spokeswoman for Taco Bell’s new Drive-Thru Diet advertising campaign for the Fresco menu, which features Ms. Dougherty’s story in TV and print advertisements, and online. The company recently began offering the menu in its drive-through kiosks, and not just inside the restaurants.

Taco Bell isn’t the only fast-food business to have jumped on the reduced-calorie bandwagon, but it is one of the few to promote their low-fat offerings so widely. Over the last few years several chains have introduced lighter menu items with less fanfare: Dunkin’ Donuts sells egg-white sandwiches. Quiznos offers a 500-calorie-and-under menu. Starbucks has panini sandwiches with 400 calories or less. In 2004, McDonald’s briefly offered Go Active! Happy Meals for Adults (complete with pedometer), but these days sells items, like a grilled chicken sandwich, without promoting their low-fat attributes.

Then there’s Subway, which, besides its 230-to-380-calorie Fresh Fit subs, claims that most of its sandwiches are low calorie, provided consumers don’t add high-fat condiments. The chain shot to diet fame 10 years ago when Jared Fogle said that he lost 245 pounds by eating Subway fare for lunch and dinner.

Hoping to repeat that success, Subway recently recruited Shay Sorrells, a contestant on “The Biggest Loser,” to be a spokeswoman. Ms. Sorrells lost 161 pounds on that show, but her weight still hovers around 315 pounds. Enter Subway, which is offering her $1,000 for every pound she loses until May, ideally with the help of its sandwiches.

The recent promotional activity surrounding the weight-loss potential of eating at places like Taco Bell and Subway has drawn the attention of experts, who have conflicting opinions on whether fast food is a healthy dieting tool.

“Fast food in and of itself is not at all necessarily a problem or insult for developmentally mature or healthy adults,” said Dr. Peter Pressman, an internist with the Navy Medical Corps in Jacksonville, Fla. “If the caloric input — regardless of composition — is not excessive, there’s no inherent physiologic evil.”

Dr. Stephen Sinatra, a cardiologist in Manchester, Conn., and the author of “The Fast Food Diet,” agreed. “With fast food you get the good, the bad and the ugly,” he said. “It’s like weaving your way through the minefield. You can step on a mine and blow up.”

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Chicken Supreme Burrito from Taco Bell.Credit
Evan Sung for The New York Times

He noted that a bean burrito provided fiber, protein and amino acids. “However, if you do the Fresco Burrito Supreme Chicken it’s got an enormous amount of sodium — 1,410 milligrams — which is a disaster,” Dr. Sinatra said.

The American Dietetic Association recommends adults consume fewer than 2,300 milligrams of sodium per day. But a study reported in the January issue of The New England Journal of Medicine suggests that consuming less salt by even a small amount can reduce cases of heart disease and stroke as much as reducing weight, lowering cholesterol and stopping smoking.

Kirsten Bibbins-Domingo, the lead author of the study and an assistant professor of medicine and epidemiology at the University of California, San Francisco, said low-fat items offered at fast-food restaurants were often salty because sodium “makes things taste good.”

She added, “When we take these other things out of food we often add back salt, and the salt is so far in excess of what would be considered a healthy quantity that it definitely counteracts any healthy benefits.”

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Some nutritionists suggest that consumers be careful of anything the fast-food business has to say about reduced-calorie, or healthy, options. “Even if they’re offering healthy fare, go into it with a wary eye — more likely they’re tricking you,” said Elizabeth Somer, the author of “Eat Your Way to Happiness,” and a registered dietitian in Salem, Ore. “The fast-food restaurants have not led the troops in healthy eating yet, so there’s no reason to believe they’re going to change their colors now.”

Morgan Spurlock, who dined at McDonald’s for a month and documented his experiences in the 2004 film “Super Size Me,” is aghast at the notion of dieting on fast-food fare. His experiment resulted in a 25-pound weight gain. “When I first heard about this, I was speechless,” Mr. Spurlock wrote in an e-mail message. “Everyone would rather take the easy way out instead of educating themselves and taking the time to eat the most nutritious food. I personally am holding out for the White Castle diet.”

Others worry that the companies might not accurately report nutritional information, which is a legitimate concern. A study published this month in The Journal of the American Dietetic Association found that the caloric content of food from 29 Boston chain restaurants and 10 frozen meals sold in supermarkets averaged 18 percent more calories than the stated values.

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Shay from “The Biggest Loser.”Credit
Trae Patton/NBC

Susan Roberts of Tufts University, the lead author of the study, planned to include some fast food and frozen foods in her 2008 book, “The ‘I’ Diet.” As research, she spent 10 days eating a variety of foods, including one fast-food meal a day, trying to lose weight. But she did not shed weight and attributes that largely to fast and commercially prepared food. “At first I thought it was just the sodium, but it went on,” said Dr. Roberts, who is planning a more expansive version of the study.

Taco Bell argues that the Fresco menu is not a diet plan. “It is not a prescriptive diet in this case, more in style with the word ‘diet’ being your overall food intake for a day,” Mr. Poetsch said.

Indeed, on Taco Bell’s Web site, a disclaimer reads: “The Drive-Thru Diet menu is not a weight-loss program,” and goes on to suggest that consumers interested in a healthier life “pay attention to total calorie and fat intake and regular exercise.”

Dr. Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale, does not object to Taco Bell’s lighter meals per se. He believes fast food restaurants should offer leaner fare, “given the health of the nation.”

“I don’t think they’re committing nutrition fraud,” he said. “KFC has some grilled chicken, and you’re better off with grilled rather than fried. If you go to Taco Bell, you’re better off having something off the Fresco Menu.”

But Dr. Brownell said marketing these foods as weight loss tools could be misleading. “Making it seem like a fast food restaurant is a place to go when you’re trying to lose weight, and making the restaurant itself seem healthier than it really is a violation of the public trust,” he said.

Ruth Carey, a registered dietician in Portland, Ore., and a consultant for Taco Bell on the Drive Thru Diet menu, said that Taco Bell had no trans fats in any of its offerings, and that four of the seven Fresco items contained 350 milligrams of sodium or less. “Also, not everyone is sodium sensitive and has high blood pressure,” she said.

Wendy Wimmer, a 38-year-old business analyst in Green Bay, Wis., for example, has low cholesterol and lost 12 pounds on a diet that included fast-food items several years ago. She quickly gained it back. “Thankfully I am blessed with crazily low cholesterol so I didn’t worry about that too much. However, I’m sure that my sodium intake was through the roof,” she said, adding that she felt “sludgy and gross all the time.”

Now Ms. Wimmer dreams about fast food that is also healthy. “I’m not talking about freaky ‘apple fries’ as a side for a Whopper, but rather a place where fresh fruit and steel-cut oatmeal is as fetishized as deep-fried French toast sticks,” she said. “If, say, Trader Joe’s decided to compete with McDonald’s, can you imagine the possibilities?”

A version of this article appears in print on January 28, 2010, on Page E1 of the New York edition with the headline: Forget Jenny Craig. Hit the Drive-Thru. Order Reprints|Today's Paper|Subscribe